1. Field of the Invention
This invention relates to a bipolar electrode which comprises an anode member and a cathode member separated from each other by a partition wall and electrically and structurally connected to each other, and which is suitable for electrolyzing an aqueous solution of an alkali metal chloride, etc., for the production of alkali metal chlorates, or alkali metal hydroxides and chlorine.
2. Description of the Prior Art
A conventional bipolar electrode is disclosed in U.S. Pat. No. 3,859,197 and has the structure shown in FIG. 1. In FIG. 1, reference numeral 1 represents a composite member obtained by explosive welding of titanium plate 4 and a mild steel plate 5. The composite member 1 is fitted in an opening of a partition wall 16 of a titanium sheet 2 and a mild steel sheet 3 so that the composite member forms a part of the partition wall 16. The outer edge portion of the titanium plate 4 of the composite member 1 is welded to an opening in titanium sheet 2, and the outer edge portion of the mild steel plate 5 is welded to an opening in the mild steel sheet 3.
The titanium plate 4 of the composite member 1 is welded to an anode member 7 with titanium as a substrate through a titanium spacer 6 welded to the titanium plate 4, and the mild steel plate 5 of the composite member 1 is welded to a cathode member 9 by means of a spacer 8 of mild steel welded to the mild steel plate 5. Thus, the anode member 7 and the cathode member 9 are connected electrically and structurally by the composite member 1 to form a bipolar electrode having an anode compartment 10 and a cathode compartment 11.
The anode member 7 is made of a mesh-like titanium substrate having formed thereon a coating of a platinum-group metal or a platinum-group metal oxide, and the cathode member 9 is formed in a grid shape.
The conventional bipolar electrode described above has the defect that the metal employed on the cathode-side (hereinafter "cathode-side metal") such as iron of the composite member has poor adhesion to the metal employed on the anode-side (hereinafter "anode-side metal") such as titanium and they tend to separate from each other physically, and that when the electrode is operated for long periods of time, titanium hydride forms at the joint portion of the composite member resulting in a separation of the metals of which the bipolar electrode is constructed. The reason is while metals suitable as the cathode member, such as iron or nickel, have a low hydrogen overvoltage at the cathode and easily permit permeation of hydrogen atoms, materials suitable as the anode substrate, such as titanium, readily form hydrides.
Hydrogen evolution in the electrolysis of an aqueous solution of an alkali metal chloride occurs according to the following two-stage reaction. EQU H.sup.+ + e .fwdarw. H.sub.(ad) ( 1) EQU H.sub.(ad) + H.sub.(ad) .fwdarw. H.sub.2 .uparw. (2)
wherein H.sub.(ad) represents adsorbed hydrogen. It is known that reaction (2) determines the rate of the entire reaction. For this reason, the surface of iron which is a cathode-side metal of the composite member is always filled with H.sub.(ad), a part of which permeates through the iron and finally reaches the portion of the composite member where the metals are joined. At this portion, the hydrogen reacts with the titanium used as the anode-side metal to form physically brittle titanium hydride and thus cause a breakage of the portion where the metals are joined to occur. Consequently, the metals are electrically insulated from each other, and the voltage between both surfaces of the composite member increases until finally the electrode becomes useless. The time which elapses until this phenomenon occurs varies depending on the current density at the portion where the metals are joined and the thickness of the cathode-side metal. For example, a composite material composed of iron having a thickness of 10 mm and titanium which are explosion-welded to each other will become useless in 1.5 to 3 years when used at a current density of 200 A/dm.sup.2.
A bipolar electrode of the type shown in FIG. 2 was devised in an attempt to remove the defects described above. A composite member 12 comprises an anode-side portion 13 made of a metal such as titanium or a titanium alloy used as a substrate of an anode member 7 and a cathode-side portion 15 made of a metal such as mild steel or an alloy of mild steel used as a base of a cathode member 9, with the portions 13 and 15 being bonded to each other with an interlayer portion 14 made of copper or a copper alloy such as brass therebetween. The portions 13, 14 and 15 are formed in a plate shape, and bonded using an explosive welding method or a frictional welding method. The portion 14 as an interlayer of the composite member 12 may be composed of two or more laminated layers.
The composite member 12 is fitted in an opening of a partition wall 16 so as to form a part of the partition wall 16. Stated more specifically, the outer edge portion of the portion 13 of the composite member 12 is welded to an opening portion of a sheet 2 of the partition wall 16, and the outer edge portion of the portion 15 is welded to an opening portion of a sheet 3. When the composite member 12 is made a part of the partition wall 16, the portion 14 made of copper or a copper alloy which becomes an interlayer of the composite member 12 needs to be formed such that it will not be exposed to the electrolyte solution.
With such a structure, the copper or copper alloy portion used as an interlayer of the composite member does not permit the permeation of hydrogen, and, therefore, hydrogen which is generated on the cathode side during electrolysis does not reach the joint surface between the interlayer and the portion made of titanium. Hence, the portion made of titanium does not separate from the portion made of copper or copper alloy at the surface thereof which is bonded. On the other hand, the portion made of mild steel has very good adhesion to the portion made of copper or copper alloy, and the portion made of copper or copper alloy does not easily form a hydride. Accordingly, the portion made of mild steel does not tend to separate from the portion made of copper or copper alloy at the surface thereof which is bonded.
However, since in the structure shown in FIG. 2, the portions 13, 14 and 15 of the composite member are bonded together by only a surface-to-surface bond, the surface-to-surface bond tends to be destroyed by mechanical factors or under severe electrolyzing conditions. Furthermore, since in the above structure the opening portion of the mild steel sheet 3 of the partition wall 16 is welded to the outer edge portion of the mild steel portion 15 of the composite member 12 so that the composite member 12 is fitted in the opening of the partition wall 16 to form part of the partition wall 16, the mild steel portion 15 after welding has no tolerance to heat deformation. Consequently, cracks occur in the welded part due to stress, or this structure tends to cause cracks to occur at the welded part due to temperature changes during electrolysis. Moreover, when cracks are present at the welded part of the portion 15 and the sheet 3, the cracks increase during electrolysis, and the catholyte solution penetrates through the cracks. This causes a destruction of the joint part of the composite member 12, and the portion 14 made of copper or a copper alloy as an interlayer of the composite member 12 is corroded. Thus, an electrically insulated condition is generated within the composite member to induce an increase in voltage.
U.S. Pat. No. 3,884,792 also discloses a bipolar electrode structure which includes an anode, a layer of an atomic hydrogen permeable base material between the anode and a cathode and a layer of a metal or a metal alloy as an interlayer which is resistant to the flow of atomic hydrogen. From the description in this U.S. patent, capped tungsten screws are used in order to secure the anode and cathode to the core, bolts typically constructed of mild steel are used for securing each pair of cathode plates, or connectors typically constructed of copper are used as electrical connectors. Therefore, this U.S. patent has the same disadvantages as set forth above and corrosion along the tungsten-capped screws tends to occur when such are used.